Morphology of Sulfide Structures in the Active Hydrothermal Fields of Indian Ocean Ridges and Its Geological Implications

The Indian Ocean ridges are shaped like the Greek letter lambda (i.e., λ), and are up to approximately 18,000 km in length with various full spreading rates (<12–80 mm/yr) including the Carlsberg, Central, Southeast, and Southwest Indian Ridges. To date, in total of 15 active hydrothermal fields have been reported at these ridges with varying intensities of activity, but the distribution and morphology of their sulfide structures, which are formed by the venting of hot fluid in cold seawater, have not been well studied. In this paper, we review the published literature and compile descriptions of sulfide structure morphology in the active hydrothermal fields of Indian Ocean ridges by combining relevant video and images. These hydrothermal fields are hosted by ultramafic rocks, basalts, or both, but the host rock is not the sole factor controlling the morphology of sulfide structures. Some hydrothermal fields contain sulfide structures sharing similar features despite different host rocks. In addition, some hydrothermal fields have large sulfide structures indicating a relatively stable tectonic environment while others only have small and short structures indicating a weak supply of materials. Nevertheless, these characteristics alone are not sufficient to demonstrate significant differences in morphology between the sulfide structures in the Indian Ocean and at the slow-spreading Mid-Atlantic Ridge. The Longqi-1 field on the Southwest Indian Ridge is unique for hosting large flanges on sulfide edifices, which contrasts with most hydrothermal fields particularly in sediment-starved areas. Regardless of the level of activity, a long-term locally stable tectonic setting and a continuous heat supply are necessary for the formation of large sulfide structures. The various morphologies in these hydrothermal fields do not correspond strictly to the tectonic settings they're located, and some local conditions such as the various permeability of the subseafloor crust could be more directly influencing the morphology of the hydrothermal structures. This review can aid in evaluating Indian Ocean ridges for mineral resources and enhance understanding of the formation of hydrothermal fields on the seafloor.